The present invention relates to a method and apparatus for converting a substance which exhibits a sol-gel change from a gel state to a sol state depending upon temperature such as an emulsion or an emulsified product used in manufacturing photographic material.
During the manufacture of photographic material it is necessary to cool gel-like substances such as photographic emulsions, matting agents or emulsified products after preparation, store them at low temperatures, and melt them as required when they are to be coated on a support. Two primary methods for melting such gel-like substances have been used in the past. One of these is a so-called batchwise method such as disclosed in Japanese Patent Publication Nos. 9495/69 and 1738/76 while the other is a melting method based on continuous control as disclosed in Japanese Laid-Open Patent Publication No. 34713/76 and U.S. Pat. No. 3,847,616.
The batchwise method is disadvantageous in that the gel-like substance tends to undergo localized overheating within a container, a long period of time is required for it to be completely melted, and the amount of the final product is small. Hence, this method is not feasible for use with gel-like substances such as photographic emulsions which undergo qualitative degradation with the passage of time.
With the latter melting method, the gel-like substance is placed on a grid-type heater in the shape of, for example, convolutions provided at the bottom portion of a melting tank, a heating medium is introduced into the grid, and the gel-like substance is continuously melted. Since this method permits the melting operation to be carried out continuously, the gel-like substance can be treated in the same vessel within a shorter period of time than with the batchwise method.
The melting method, however, has the following disadvantages.
(i) The melting speed must be adjusted by varying the flow rate, temperature or pressure of the heating medium introduced into the grid. Hence the melting speed cannot be markedly increased because there are limits to the permissible ranges of these parameters. For example, if the temperature is raised to too high a level, the properties of the gel-like substance are adversely affected. The melt-treating time is therefore difficult to shorten.
(ii) For the same reasons, the margin of control of the melting speed is narrow, and its control is difficult.
(iii) Since the heater is fixed to the bottom of the tank, it is troublesome to wash the tank, especially the undersurface of the heater, and a long period of time is required for a complete washing of the tank.
Accordingly, it is an object of the invention to provide a method for melting a gel-like substance which eliminates the aforesaid disadvantages of conventional melting methods and in which the speed of melting the gel-like substance is greatly increased, the margin of control of the melting speed is increased, and the melting tank can be easily washed.
Another object of the invention is to provide an apparatus for practicing the aforesaid method.